I believe the colours fall outside the GW range, since GW is quite limited compared to the likes of Vallejo. But if i were to attempt it, i guess i would go with Khaki + Codex maybe Fortress, do a test run first, and then add maybe bleached little by little to the mix and then start swapping the bleached bone in the mix with a little white. It would probably be worth making a mix pot of khaki+codex for your base colour if your trial run works out since it would be the base colour for mixing with.
The problem is the colour is very similar to what my argus i recently mostly painted is. But i cant tell by unnatural light what colours i used and ill be damned if i cant remember either.
This method might not get you that colour there, but it is definitely how i would try and get it using the GW core range. (There might be a better way with the foundation range, but i have next to 0 experience with it)
As for layering? Watery paint with thin layers, not too much paint on the brush as you will just flood the model instead. I find if i am trying to paint really nice models, if you damp the area around where you intend to paint on with a damp brush, it stops your watery paint forming that horrible edge sometimes.
The method i normally use because i am lazy is simply, paint a layer, add a little bit more colour and paint slightly in from the last area. Thats the basics and thats what i normally do too. Simple and effective, why mess with what works alright at its basic level?
Oh and most importantly, patience. I normally get bored and rush it at the end and ruin what was looking awesome. But patience and not changing the colour too much between layers is key for a nice paintjob.